Zeolites are microporous, aluminosilicate minerals. They possess well-defined pore structures and have high specific surface area. These features have been exploited in a number of commercial applications including adsorbents and catalysts.
Zeolites are classified according to their framework types as defined by the International Zeolite Association (IZA) Commission on natural zeolites. The classification was first proposed by Meier and Olson in 1970 and has gained wide acceptance in the zeolite community. A framework type describes the connectivity of the tetrahedrally coordinated atoms of the framework in the highest possible symmetry. A three letter code is assigned to confirmed framework types by the Structure Commission of the IZA according to rules setup by the International Union of Pure and Applied Chemistry (IUPAC) Commission on zeolite nomenclature. As of January 2008, 175 unique zeolite frameworks have been identified, and over 40 naturally occurring zeolite frameworks are known.
Zeolite minerals species are not normally distinguished on the basis of the framework's Si/Al ratio. An exception is made in the case of Heulandite and Clinoptilolite, where Heulandite is defined as the zeolite minerals having the distinctive framework topology of Heulandite (HEU) with Si/Al ratio<4.0. Clinoptilolite is defined as the series with the same framework topology with the Si/Al ratio of ≧4.0. Depending on the extra cations present in the zeolite, the framework may have a suffix, for example Heulandite-Ca, Clinoptilolite-Na, and the like.
Clinoptilolite has been previously used as gas separation media. For example, U.S. Pat. No. 5,116,793 uses Clinoptilolite to separate molecules from a feed stream where one of the molecules has the tendency to adsorb strongly to the zeolite than other molecules. After the zeolite is loaded to a desired extent with the adsorbed component, the conditions of the zeolite are varied, for example in temperature or pressure, at which the zeolite desorbs the adsorbed component. The present invention uses the zeolite as a filtering media to separate Hydrogen from a syn-gas stream.
Clinoptilolite and Heulandite are normally mined because they are difficult to synthesize. There are very few examples where Clinoptilolite and Heulandite have been synthesized using seed crystals. Hence, economics are quite favorable for industrial use if the zeolite is mined rather than synthesized. There are mainly two reasons why mined zeolites have not been used for industrial applications. The first is the high level of impurities in the mined rocks, and the second is high macroporosity normally present in mined zeolites.
The first problem, i.e., impurities, is normally addressed via surface treatments, re-crystallization or ion-exchanging processes. This is undertaken after crushing the rock and before membrane fabrication. The second problem, macroporosity, is addressed by examining the mined rocks. There are very few mines in the world that yield mined zeolites with little to no macroporosity. For example, the clinoptilolite material obtained from Castle Mountain in Australia, Manery Creek mine in the Mt. Kobau area of British Columbia, Canada, and the Saint Cloud mine in New Mexico, USA appear to be unique deposits where extremely high geological pressures have been applied to essentially remove all the macroporosity from the source rocks. The bulk density of the zeolite rock is a good indicator of its macroporosity. A good indication that the zeolite rock has been naturally compressed to the point where the rock has essentially no macroporosity is when the bulk density of the zeolite rock is close to the crystal density of zeolite. The bulk density of rocks mined from Castle Mountain and Manery Creek mines are between 2.1 to 2.2 g/cc and the crystal density of Clinoptilolite is 2.2 g/cc.
Due to their well-defined pore structure, zeolite materials can be used for gas separation. A zeolite separation membrane must have both high flux and selectivity to be practical. Fabrication of practical zeolite membranes, including supported zeolite membranes, has long been a goal of separation and catalyst science. The source zeolite for membranes can either be natural or synthesized. Synthetic zeolites have a greater uniformity and purity than natural zeolites. Hence, synthetic zeolites are more commonly used industrial applications where purity is essential. For example, zeolites are widely used industrially as they exhibit shape-selective catalytic properties.
There are currently three general approaches for fabricating zeolite membranes. The first approach involves fabrication of zeolite membranes directly from natural deposits. In this case, rock fragments are machined into membrane structures, as shown in FIG. 9. These membrane structures have shown promising gas separation performance under laboratory conditions. However, this approach is limited by the quality and size of the starting rock material. These have high defect and crack density with macroporosity that make them unsuitable as membrane materials. Due to the nature of the process, they are form factor limited. Typically, zeolite membranes are fabricated or disposed upon a porous substrate. As such, the economics of scale up are also not favorable.
The second approach involves the fabrication of a continuous zeolite layer on a porous support. Currently, there is considerable prior art that describes the preparation of supported zeolite catalysts; specifically many different types of zeolites have been deposited on various supports. This is a “bottom-up” process that generally involves hydrothermal nucleation and growth of zeolite particles from a precursor solution under conditions amenable for the zeolite to form, as shown in FIG. 10. Zeolite particles nucleate and grow on the surface of the porous support. A continuous zeolite layer is eventually formed when the growing zeolite particles impinge upon each other.
This hydrothermal synthesis is normally done under high pressures, temperatures between 150° C. to 250° C., and time between 6 hours to one week. In some cases, interstitial voids remain between the zeolite particles that can lead to crack propagation when the membrane is mildly stressed. These voids act as defects that allow fluid flow to bypass the porosity in the zeolites, compromising the performance of the zeolite membrane. Conversely, extended growth periods can lead to thick layers that have low permeability and also lead to delamination failure of the membrane. The hydrothermal synthesis generally forms zeolite columns that are parallel to the growth direction, as shown in FIG. 10. Additionally, this hydrothermal process is difficult to economically scale-up.
A variation of this second approach is described by Choi et al., [Ind. Eng. Chem. Res., 2007, 46, 7096-7106], where the membrane is fabricated in numerous layers. The composite membrane as shown in the schematic in FIG. 11 is comprised of MCM-22 zeolite that is grown hydrothermally and deposited alternately with mesoporous SiO2 on a porous support. The deposition itself is not hydrothermal, but using evaporation induced the self-assembly process. Choi et al. attributed the improvement in gas selectivity to the oriented platelet morphology of MCM-22, as shown in FIG. 11 and the number of alternating layers. More layers help gas selectivity, but also decreases flux. Currently, this approach has only been demonstrated for large pore zeolite flakes in a mesoporous silica matrix. This approach is time consuming (expensive) and is prone to fail due to delamination, leading to poor manufacturability and high defect density. In addition, there are no known reports of structures comprising zeolite inclusions that span the entire thickness of the composite structure.
A third approach for the fabrication of zeolite membranes involves forming a mixed-matrix membrane, wherein zeolite particles are disposed in a continuous polymer matrix. Zimmerman et al., (Journal of Membrane Science 137 (1997) 145-154) fabricated a mixed-matrix zeolite-polymer membrane to separate oxygen and nitrogen. Membranes produced using this approach can exhibit superior gas separation performance, relative to a polymer membrane without the zeolite material. However, there are some key disadvantages of this approach. The gases have to diffuse through a continuous path of several zeolite particles. The flux will decrease significantly if the gases diffuse though the less-permeable polymer material, as shown in FIG. 12. In addition, the use of these membranes is limited to conditions the polymer membrane is capable of surviving. Further, similar to the second approach, there are no known reports of structures comprising zeolite inclusions that span the entire thickness of the composite structure.
As described above, zeolite membranes are currently limited by the ability to produce structures substantially free of the defects that compromise separation performance, but also possessing significant permeability to fluids. In addition, there is a need for structures that are amenable to scalable and low-cost manufacturing methods that can produce zeolite membranes with favorable economics, particularly for gas separation applications.